1. Field of the Invention
The present invention generally relates to image sensing systems and more particularly relates to a linear image sensor with tricolored photodetectors.
2. Description of the Related Art
There are many applications that need an image scanning system to convert a scanning object to an electronic format that can be subsequently analyzed, printed, distributed and archived. The electronic format is generally an image of the scanning object. A typical example of the image scanning system is a scanner and the scanning object is a sheet of paper from a book or an article. Through the scanner, an electronic or digital image of the paper is generated.
An image scanning system generally includes a sensing module that converts a target optically into an image. The key element in the sensing module that converts the target optically to the image is an image sensor comprising an array of photodetectors responsive to light impinged upon the image sensor. Each of the photodetectors produces an electronic (charge) signal representing the intensity of light reflected from the target. The electronic signals from all the photodetectors are readout and then digitized through an analog-to-digital converter to produce digital signal or image of the target.
One of the image sensors used in the image scanning system is a linear sensor comprising three lines of photodetectors respectively superimposed upon with selectively transmissive filters in registration with each of the photodetectors. FIG. 1 illustrates a sample linear image sensor 100 comprising three rows 102, 104, 106 of photodetectors, each row being behind one kind of transmissive filter sensitive to a particular spectral region. For example, all photodetectors in row 102, 104 and 106 are respectively superimposed upon with a red filter, a green filter and a blue filter. Therefore image sensor 100 produces three electronic signals simultaneously when photodetectors in image sensor 100 are exposed to a scanning object illuminated by a white illumination source.
One of the noticeable problems associated with image sensor 100 is the requirement of auxiliary memory line buffers and processes to repackage electronic signals to precisely represent each registered image pixel. Because three photodetectors in each of the three rows are not physically registered to one scanning dot due to gaps among rows of photodetectors and associated circuits respectively supporting the photodetectors, a pixel registration process has to take place before the electronic signals or subsequent digital signals result therefrom can be used.
There have been some efforts to minimize the registration problem. One method is to reduce the gap between two rows of photodetectors by reducing the size of the photodetectors so that corresponding photodetectors in each of the three rows can be barely registered. However, a significant reduction of the photodetector size can introduce image sensitivity problems. Therefore there is a great need for a color image sensor that presents no registration problems and requires minimum memory buffers and processes to output a satisfactory type and quality of color image signals.
The present invention has been made in consideration of the above described problems and needs and has particular applications to image sensors used in scanners, facsimile machines and photo copy machines. As the features and advantages described below, the present invention can be advantageously used in portable scanning devices.
An image scanning system generally includes a sensing module that converts a target optically into an image. The key element in the sensing module that converts the target optically to the image is an image sensor. When an image scanning system employs the disclosed color image sensor, the memory buffer and processes used to repackage electronic signals from the photodetectors can be eliminated, scanning speed can be improved. In addition, when a black-and-white scanning object is scanned, the image resolution is inherently doubled due to a unique arrangement of the photodetectors in the image sensor.
According to one aspect of the present invention, the disclosed color image sensor comprises a row of photodetectors, every other one of the photodetectors being of a triangular-like shape and every another one of the photodetectors being of an upside-down triangular-like shape; a filter made of individual filter elements superimposed in one-to-one registry on the photodetectors; and wherein every other one of the filter elements has a first spectral transparent characteristic, every another one of the filter elements has one of a second spectral transparent characteristic and a third spectral transparent characteristic.
Objects and advantage together with the foregoing are attained in the exercise of the invention in the following description and resulting in the embodiment illustrated in the accompanying drawings.